Zbib bibliography is stored in your browser's local storage as explained here. It's also possible to store a version of a bibliography on Zotero servers using Link to this version button. If you've used that, then the link that zbib offered can be used to recover the bibliography, otherwise your bibliography might unfortunately be gone.

Zbib is not really meant for anything that takes "months of work", it's more for making quick, one-off bibliographies. For any serious work you should consider installing Zotero which is much more powerful and persists your citations in a database on your computer (and optionally in a cloud).

I transfer a rtf file on my MacBook and I can see an "url blob" when I watch information on that file, I don't know if that can be useful for you..

If you cannot recover your work in any other way but have an .rtf file exported from zbib, you might be able to use anystyle.io to process formatted references. This however is not guaranteed to work and will most likely require some manual corrections.